The disclosure relates to gas turbine engines. More particularly, the disclosure relates to thermal barrier coatings for gas turbine engines.
Gas turbine engine gaspath components are exposed to extreme heat and thermal gradients during various phases of engine operation. Thermal-mechanical stresses and resulting fatigue contribute to component failure. Significant efforts are made to cool such components and provide thermal barrier coatings to improve durability.
Exemplary thermal barrier coating systems include two-layer thermal barrier coating systems. An exemplary system includes a NiCoCrAlY bond coat (e.g., low pressure plasma sprayed (LPPS)) and a yttria-stabilized zirconia (YSZ such as 7YSZ) thermal barrier coat (TBC). While the TBC layer is being deposited or during an initial heating cycle, a thermally grown oxide (TGO) layer (e.g., alumina) forms atop the bond coat layer. As time-at-temperature and the number of cycles increase, this TGO interface layer grows in thickness. U.S. Pat. Nos. 4,405,659 and 6,060,177 disclose exemplary systems.
Exemplary TBCs are applied to thicknesses of 0.05-1.0 mm and can provide in excess of 300° F. temperature reduction to the base metal (e.g., a Ni- or Co-based superalloy). This temperature reduction translates into improved part durability, higher turbine operating temperatures, and improved turbine efficiency.
U.S. Pat. No. 5,652,044, (the '044 patent) the disclosure of which is incorporated by reference in its entirety herein as if set forth at length, discloses a system for forming alternating TBC layers by switching between a plasma assisted physical vapor deposition (PA-PVD) mode and a non-plasma assisted physical vapor deposition (PVD) mode. Due to the active role of ions in the PA-PVD processes they often are also forms of ion-enhanced PVD (IE-PVD). The definition of PA is more general because other plasma species (other than ions: chemically active excited atoms and molecules, radicals, etc.) may also play important role in PA-PVD to obtain desired composition coatings (as will be clear from below). So, the PA-PVD process of the '044 patent is more fully defined (for purposes of discussion herein only) as PA(IE)-PVD. During PA(IE)-PVD stages, plasma ions bombard the surface of coated TBC and introduce modifications into TBC microstructure, which affect TBC thermal conductivity. Namely, sharp transitions from the microstructure of one deposition mode to the microstructure of the other deposition mode strongly influence the TBC thermal conductivity due to scattering of providing thermal conductivity phonons and photons on the transitions (which occurs at the interfaces between the microstructures). The alternating stages are of approximately equal duration. In one experiment, the layers of both modes were 2 μm; in another, the PVD layers were 2 μm and the PA(IE)-PVD layers were 1 μm. The equal thickness combination was identified as having a lower thermal conductivity than the unequal thickness combination and the respective single mode coatings.
Especially when used in sandy desert environments, engine components are subject to a particular form of fouling/damage known as molten sand attack or CMAS. CMAS is an abbreviation for “calcium-magnesium-aluminum-silicon (silicate)”. Specific CMAS oxides include CAO, MGO, Al2O3, and SiO2. CMAS components may form a eutectic with a relatively low melting point (e.g., approximately 1240 C). The molten CMAS material infiltrates into porous coatings (e.g., between the columns of columnar ceramic). This can alter the chemical composition of the coating and/or cause structural failure of the coating. Efforts to address CMAS have centered on improved barrier coatings. For example, U.S. Pat. Nos. 5,660,885, 5,871,820, 5,914,189, 6,720,038, 6,627,323, 6,465,090, all reference coatings relative to CMAS.